Spider glides from the sky, can choose where it lands

The gliding spider, which hunts in the treetops, is thin and light.
Stephen Yanoviak/University of Kentucky

A spider native to the forests of South America has been discovered to possess a magnificent and unique trick: It is able to lightly glide and steer its descent as it drops from a tree down to the ground.

Found in Panama and Peru, the hunting spider, which belongs to the genus Selenops, is the only known spider that can control its direction in the air. But it doesn't do so for hunting purposes. Instead, the ability to safely leap out of a tree is an escape tactic, used to flee from predators and other threats.

In that, it joins several other animals: flying squirrels, gliding snakes and other crawling insects such as ants and bristletails.

"My guess is that many animals living in the trees are good at aerial gliding, from snakes and lizards to ants and now spiders," said Robert Dudley, University of California at Berkeley professor of integrative biology and the co-author of a new study of Selenops. "If a predator comes along, it frees the animal to jump if it has a time-tested way of gliding to the nearest tree rather than landing in the understory or in a stream."

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Dudley and University of Louisville Professor of biology Stephen Yanoviak have been studying tropical forest gliding insects for over a decade. Their work began when they noticed that a particular type of ant always managed to land on another tree when knocked off a branch.

The pair has been throwing arthropods from treetops ever since to find out which ones can glide.

So far, they have found that ants, bristletails and now this spider are the only adult arthropods that can glide. However, the immature wingless forms of several winged adult insects, such as cockroaches, mantids, katydids and stick insects, can also glide.

To test and study the Selenopsspider's gliding capabilities, Dudley and Yanoviak threw 59 individual spiders from a height and observed how they fell. The spider, which measures about 5cm (2 inches) across, is very thin and light. To steer, it spreads its legs wide, making use of lift and drag to land where it wants. It can also flip upside down, and then right itself in mid-air.

"This study, like the first report of gliding ants, raises many questions that are wide open for further study," Yanoviak said. "For instance, how acute is the vision of these spiders? How do they target a tree? What is the effect of their hairs or spines on aerodynamic performance?"

These answers could help provide some insight into how flight developed -- Dudley believes that controlled gliding could have preceded the evolution of wings. But the answers to how the spider does what it does could also help develop robots that can turn themselves right-side-up when falling, he said.